Thermostat to control new Burnham Alpine 105 boiler

I recently took advantage of the Mass Save HEAT loan and converted from oil to gas. I replaced my old Weil-McLain boiler and Amtrol indirect water heater with a Burnham Alpine 105 and Superstor Ultra indirect water heater. The one component of the system that wasn't replaced was the thermostat in the living space. My current Honeywell C***00A is about 9 years old and I would like to replace it. I asked my installer and he said programmable units are of limited value with high efficiency boilers since the outside sensor controls the flame size. I called Burnham and they recommended a non-programmable set to a comfortable setting and leave it. They thought this would be the most cost effective way to go. This seems like a totally new mind set since we've been told over and over to go programable with a 9 degree setback to save energy. I'm lost. Should I keep my old Honeywell and set it to 68 deg. all the time? Replace it with a some "smarter" unit that works well with the Alpine? Get a simple manual thermostat? I want to save money and utilize the technology of the Alpine!

It won't be an off-the-shelf from the hardware store. You could check the honeywell website. They make full-featured stats like that. One good idea is simply to ask Burnham for a recommendation. Keep in mind that such an item will run you about 2 bills.

You will save some money, but it may not be as much as you think. I like the Honeywell units since many of them you set the time you want the house at a certain temp, and if it's a mild day, it may start 15-minutes beforehand, or it may start an hour or more before...it depends on how fast the system cooled off -IOW, it learns the house and the heating unit's capability. But, if the new boiler was sized well, on a really cold night, it may not save much of anything, as it would take all night to be able to provide the set time.

With dumber thermostats, you decide when to return the house to temp...say you want it warm at 0630, you might set it to turn on at 0600. On a mild day, that may be fine, but on a cold day, it may still be cold; the Honeywell eliminates that problem for the most part.

Modulating condensing boilers usually hit their max efficiency at about 1/4-1/3 of full fire, and efficiency ALWAYS increases as the temp of the water entering the boiler returning from the radiation/heat-emitters falls. When using deep setbacks with a programmable T-stat you have a couple of things working against you-

A: The firing rate on the boiler is higher during the recovery ramp from setback, reducing combustion efficiency during the recovery

and

B: In order to recover from setback at a reasonable rate, the outdoor reset curve has to be set to a higher temperature, which increases the return water temps, reducing combustion efficiency across the entire operating range.

With a non-modulating boiler you might save 5-7% with setback, maybe 10%-15% if you live in the leakiest & least insulated house in Massachusetts. But with a mod-con setting it back 9F would increase, not decrease fuel use.

With a modulating-condensing boiler running constant temp, but with the outdoor reset curve finely tweaked to the load it will use LESS fuel than using it with even 5F overnight setbacks. Read the manual, learn how to adjust the boiler's reset curve, and keep lowering it until it doesn't quite keep up, then judiciously raise it.

BTW: The -105 is oversized for most mid-sized houses in MA, which may lead to short-cycling efficiency losses (and boiler wear & tear) when running at low temp, depending on the amount & type of radiation you have, made worse if it's broken up into many zones. These are issues that should have been addressed in the re-design of the system for the low-mass modulating boiler with outdoor reset control, but all too often isn't. (With 96,000BTU/hr peak output it's nearly 3x oversized for my ~2400' house + 1500' of semi conditioned basement in Worcester- a 1920s antique with R-values & U-values way sub-code for newer construction.) The minimum-fire output of this beast is well over half my heat load at 0F outdoor temps, and while it would modulate OK for the coldest weeks of January it would be cycling the rest of the time, and with insufficient thermal mass or radiation to not short-cycle when it's 30-40F outside the condensing efficiency of the lower temp could get chewed up by cycling losses. It's possible to fix these issues to save the boiler and increase efficiency if that turns out to be the case with your house.

FWIW, with my boiler, I found that comfort was much improved at essentially no increase in cost by not setting the temp back at night. This agrees with Dana's assessment, too. Now, if I'm going away for a couple of days, then I may set it back. Once all of the furniture and interior walls have dropped to the setback temp, it takes a long time before they reach room temp, so even if the air temp is normal, it feels like the furniture and walls are sucking heat out of you...much more comfortable to have them at a nice stable temp. Depending on your insulation levels, you could experience that even with an overnight setback, but it obviously wouldn't be as extreme as setting back for days. Some of the thermostats may have anti-short cycle logic, and some allow you to set the hysterisis, which may help some. But, long burns at low temps are more comfortable and wallet friendly.